The present invention relates to aqueous emulsion paints which contain urea-formaldehyde condensation polymers as white pigments. Aqueous emulsion paints which contain special urea-formaldehyde polymers having a specific surface area of 15 to 100 m.sup.2 /g and a bulk density of less than 100 g/l as white pigments in addition to TiO.sub.2 have already been described in U.S. Pat. Nos. 3,737,404 and 3,836,495. A conspicuous feature of these emulsion paints is that the amount of urea-formaldehyde white pigment is only about 2 to 20% by weight, based on the total volume of white pigment. The reason for the limitation is evidently that further replacement of TiO.sub.2 by the urea-formaldehyde white pigment (i.e. raising the concentration to above 20% by weight) results in such a pronounced thickening and unfavourable change in rheological properties that the viscous mass is no longer suitable for coating purposes. The urea-formaldehyde polymers described in the patent specifications referred to above thus act as extenders for titanium dioxide. In view of this prior art, the assumption necessarily was that it will not be possible to prepare emulsion paints of high hiding power and with a high degree of whiteness in which urea-formaldehyde polymers do not act as extenders but as white pigment, and are able to replace the titanium dioxide completely or to a great extent in such paints. However, substitution of organic white pigments for titanium dioxide would be desirable for the following reasons. At the present time, a shortage of high-grade titanium ores (rutile, ilmenite, brookite, anatase) is already perceptible and their almost complete exhaustion is foreseeable. To save resources it seems logical to preserve titanium ores chiefly as metallurgical raw materials instead of processing them to white pigments which are exposed to weathering and rapid wear in paints. On the other hand, the basic raw materials for urea-formaldehyde white pigments (earth gas, petroleum, coal) seem ensured for much longer. In addition, a number of technical advantages make it appropriate to substitute urea-formaldehyde pigments for TiO.sub.2 in emulsion paints. The high specific weight of TiO.sub.2 of 4.26 g/cm.sup.3 not only makes it necessary to add special ingredients in order to prevent sedimentation in the emulsion paint, but also increases the specific weight of the paint. As the object of applying a coat is to cover a specific area with a specific volume of paint, it is possible to coat a larger area with a specifically lighter paint which contains a urea-formaldehyde white pigment having a specific gravity of d.sub.20 =1.40. If it is desired to obtain a coherent film after drying, pigments and fillers may not exceed a specific percentage of solid pigment by volume in the film. This percentage is known as the critical pigment volume concentration. Because of the specific weights, 330 g of urea-formaldehyde white pigment take up the same volume as 1 kg of rutile, or 1 kg of urea-formaldehyde takes up the same volume as 1 kg of TiO.sub.2 +1.25 kg of CaCO.sub.3. This last substitution results in paints having the same hiding power and whiteness as illustrated in the working Examples. As urea-formaldehyde white pigments can be obtained very much more cheaply than TiO.sub.2, a substantial cost advantage accrues to the paint manufacturer.
It is, of course, not possible to manufacture urea-formaldehyde pigments which have as high a refractive index as that of titanium dioxide (2.6-2.9). It is therefore all the more surprising that it is possible to obtain emulsion paints of comparable hiding power and whiteness with urea-formaldehyde pigments which have only a refractive index of 1.65, i.e. similar to that of the polymer binders. It has been found that this is determined by specific particle and agglomerate structures which can be described by the parameters: particle diameter, pore volume and light scattering coefficient.